The recording density of a Hard Disk Drive (HDD) is about 1 Tbit/inch2 as of 2014. The recording density increases every year, and the rate of increase until several years ago is about 40% annually, and in recent years, the rate of increase continues at about a 15% annual rate. An increase in recording density is realized by micronizing the size of magnetic particles. When the size of the magnetic particles becomes smaller, magnetization of the magnetic particles becomes thermally unstable, and it becomes impossible to retain recorded information for a long period of time. In order to prevent such a problem from being caused, it is necessary to enhance the magnetic field intensity needed to reverse the magnetization of the magnetic particles, i.e., it becomes necessary to use a magnetically hard magnetic material. However, when magnetic particles which are extremely magnetically hard are used, it becomes difficult to rewrite recorded information. In order to increase the recording density, it is necessary to simultaneously overcome three mutually contradictory technical walls, i.e., micronization of magnetic particles, use of thermally stable magnetic particles for long-term retention of information, and easy rewriting of recorded information. The three walls are called the trilemma of HDD recording density improvement.
As a magnetic recording technique aiming at a breakthrough in the trilemma of HDD recording density improvement from the viewpoint of easy rewriting of recorded information, the high-frequency wave assisted magnetic recording (also called the microwave assisted magnetic recording (MAMR)) is known. The high-frequency wave assisted magnetic recording is a technique for facilitating writing of information by locally applying a high frequency magnetic field having a frequency near the resonance frequency of a magnetic recording medium to the magnetic recording medium to thereby resonate the magnetic recording medium, and temporarily lower the coercive force thereof (see, for example, U.S. Pat. No. 6,011,664). The point of this technique is to efficiently superimpose as large a high frequency magnetic field as possible upon the recording magnetic field.
As a method of efficiently generating a high frequency magnetic field, a method utilizing a magnetization oscillation element based on spin torque is disclosed (see, for example, JP-B 4050245 and US2008/0019040). The magnetization oscillation element based on spin torque has generally been called a spin-torque oscillator (STO) since about 2006. The STO can generate a high frequency magnetic field resulting from magnetization oscillation near the element by passing a DC current, and hence expectations are placed on the STO as an efficient high-frequency magnetic field generation source in the MAMR.
Regarding the MAMR using the STO, engineers concerned with the technique thereof are carrying out design/research and development while paying attention mainly to the following four points: (1) A rotational direction of a microwave magnetic field effective for the MAMR exists (an effective microwave magnetic field component has a rotational direction identical to the direction of the precession movement of recording medium magnetization), and thus designs that take the direction into consideration are desired. (2) It is desirable that a direction of an STO drive current be a fixed direction irrespective of a direction of a magnetic field from the main magnetic pole. (3) It is desirable that designing be carried out in such a manner that a distance between the main magnetic pole and write point is as short as possible in order to make the magnetic field from the main magnetic pole as strong as possible. (4) It is desirable to cause magnetization oscillation in the STO so that the microwave magnetic field intensity becomes as strong as possible.
As a high frequency assisted magnetic recording head which generates a microwave magnetic field effective for the MAMR of point (1) above, and in which the STO drive current has a fixed direction of (2), an assisted head provided with an STO of a “perpendicular free layer+in-plane free layer” type is disclosed in JP-B 5172004. In JP-B 5172004, a high-frequency wave assisted head in which an STO of a “perpendicular free layer+in-plane free layer” type is provided between the main magnetic pole and counter magnetic pole in such a manner that the constituent elements are arranged in the order of the main magnetic pole, perpendicular free layer, in-plane free layer, and counter magnetic pole (FIG. 15A of JP-B 5172004), is described as a configuration of a desirable high frequency wave assisted head. In JP-B 5172004, it is stated that it becomes possible, by using this assisted head, to realize an information transfer rate exceeding 2 Gbit/s in magnetic recording to which microwave assisted recording realizing a recording density exceeding 1 Tbit/inch2 is applied.
However, the assisted head disclosed in JP-B 5172004 has disadvantageous points with respect to the aforementioned points to note (3) and (4). Regarding point (3), there is a problem that a perpendicular free layer (of which a description stating that the perpendicular free layer hardly contributes to the high frequency wave assisted magnetic field is given) is provided between a main magnetic pole and in-plane free layer, and thus a distance between the main magnetic pole and write point becomes longer by a length corresponding to at least a film thickness of the perpendicular free layer, and the design freedom relating to the distance between the main magnetic pole and write point is limited. Regarding point (4), two types of free layer magnetization of the perpendicular free layer and the in-plane free layer rotate with a phase difference of 180° held between them, so there is a problem that high-frequency magnetic fields resulting from these two types of free layer magnetization cancel each other out. The degree of the cancellation becomes more conspicuous with an increase in the distance between the high frequency magnetic field application point in the magnetic recording medium and STO. Incidentally, a technique of applying a high frequency wave assisted recording head using an STO to three-dimensional magnetic recording (see, for example, WO2011/030449) is disclosed. In the case of the three-dimensional magnetic recording, the distance between the high frequency magnetic field application point and STO becomes longer in a thickness direction of the medium. Accordingly, the STO in which two types of magnetization rotate with a phase difference of 180° held between them is disadvantageous to the application of the three-dimensional magnetic recording because the high frequency magnetic fields cancel each other out.